In key telephone systems, each of the conditions of a line involved in a call is indicated by a specific visual signal at the line lamps representing that line. By sensing the electrical condition generating that lamp signal, a secondary indication of that condition may be made to trigger an output.
In key systems, the line lamp of a busy signal on a phone is held operated in a steady condition, the line lamp of a phone being rung is flashed at lamp flash rate and the line lamp is winked at a wink rate when the line is in a held condition. A number of ways of sensing the line condition are possible. One approach is to sense the voltage on the line for ringing (intermittent AC), closed loop (low resistance DC line) or hold (high resistance bridged DC across the line).
However, detecting these conditions and responding promptly to a change of the condition requires voltage sensing apparatus which may materially add to the cost of the apparatus in order to provide the necessary degree of sensing. Another approach would be to sense the line circuit relays are operated, since different combinations of relays are operated for each condition. Additional contacts on each relay, slave relays or considerable additional circuitry would be required to implement this form for sensing.
In addition, it has been found that there exists a need for the calling party to know that he has been placed on hold and not disclosed from his call. This is accomplished by connecting a source of audio to the telephone line during the period of time that the calling party has been placed on hold.